The ultrasonograph has been used as a non-invasive, X-ray radiation free, and more convenient diagnostic tool than X-ray, CT (Computed Tomography), or MRI (Magnetic Resonance Imaging). Although CT and MRI provide images of high quality, they take up much space and are more expensive. Also, X-ray examination is invasive because of its X-ray radiation.
The ultrasonograph is said to provide images of low quality because its depth of penetration decreases with increase in frequency of an ultrasonic wave used to obtain of high resolution. For example, an ultrasonic wave with a frequency of 3 to 10 MHz is used in the conventional ultrasonic imaging system. In this case, depth of penetration is approximately 3 to 15 cm. It is difficult to obtain a sufficient resolution in the order of some 10 microns using an ultrasonic wave of such low frequency.
To obtain a high resolution in the order of 10 microns, it is necessary to use an ultrasonic wave with a frequency greater than 100 MHz; however, depth of penetration at this high frequency is less than approximately 1 mm, making it impossible to obtain images of deep tissue using conventional ultrasonic imaging systems.
Accordingly, to diagnose diseases with high accuracy, it is necessary to use such high frequency and to place the probe close to the region of interest. Therefore, an intraductal system capable, for example, of transesophageal scanning, in which the probe is inserted through the esophagus; or intravascular ultrasound (IVUS), in which the probe is inserted through a vessel, is employed.
The transesophageal approach, in particular, transesophageal echocardiography (TEE) is suited for examining organs to which the transcutaneous ultrasonic wave is not applicable because of interference of the ribs and lungs. On the other hand, intravascular ultrasound (IVUS) is a medical imaging methodology using a thin probe inserted into a blood vessel to diagnose the tissue. Examination by TEE is conducted by inserting a rotary probe placed at the tip of an endoscope into the esophagus for transmitting and receiving the ultrasonic wave. IVUS is performed by inserting a thin catheter for transmitting and receiving an ultrasonic wave into the vessel, but it is invasive and requires extra caution when electricity is used in the body.
Conclusion: these modalities have lost convenience and non-invasiveness.
Generally speaking, because established diagnosis is not obtained by ultrasonic imaging alone, ultrasonic imaging is carried out after CT or MRI examination. To make a definite diagnosis of the tissue, a specimen is obtained by biopsy under ultrasonic wave guidance and is processed for microscopic evaluation. This procedure is considered reliable for establishing a diagnosis, and enjoys widespread use; however, the results may require several days. Surgery therefore can not be carried out, even when abnormal findings are reported by the examination, until the established diagnosis is obtained. Further, because pathological examination of the resected slice obtained under laparotomy can not be carried out immediately, a follow-up operation is required. Laparotomy is sometimes interrupted until the pathological diagnosis is established, placing severe stress on the patient, who is kept waiting with an open abdominal incision.
To obtain an accurate diagnosis, an ultrasonic apparatus for the industrial use is tried for the medical use. The apparatus operates in the range of tens to hundreds of megahertz, a considerably higher frequency range than that of conventional ultrasonographs, in which several megahertz to tens of megahertz is used.
A focusing device is used in ultrasonic microscopy to focus the ultrasound beam. Because ultrasonic microscopy allows a pathologic diagnosis of the undyed specimen, results are available sooner than with optical microscopic methods, which require a dyed specimen. Ultrasonic microscopy has been considered for medical diagnosis, for which biopsy is performed to acquire the specimen, which is located on the path of the ultrasound beam. Because of the complexity of the procedure, this method is used only for the limited case in which time is a critical consideration.
A puncture needle-type ultrasonography was developed as an ultrasonic method to facilitate tissue diagnosis based on B-mode images [Japanese Unexamined Patent Application Nos. H02-107238 (reference 1), and H02-107239 (reference 2)]. In these apparatuses, an ultrasound probe is placed inside an outer hollow needle, and the needle is inserted into body tissue. The probe can be placed close to the region of interest, and can acquire images of high quality from deeply situated tissue using high-frequency ultrasound.
According to references 1 and 2, however, the puncture needle itself is a probe, and the ultrasound transducer is placed in the needle, necessarily increasing the diameter of the needle. The needle is 3 to 4 mm in diameter even when a small transducer is used. Use of a needle of such large diameter for puncture places the patient under a heavy burden and loses the advantage of non-invasiveness.
Accordingly, the transducer must be small enough to be mounted in the probe. However, a smaller transducer transmits less ultrasonic energy, and the resulting poor signal-to-noise ratio (S/N) precludes imaging deep tissues. Producing adequate ultrasonic energy with a small transducer requires high voltage, which demands extra caution for patient safety and sacrifices the convenience of the procedure.
Japanese Unexamined Patent Application No. H11-206759 describes a small needle-type ultrasound probe and an ultrasonic microscope. The apparatus uses an ultrasound beam of high frequency, around 100 MHz, and is used to diagnose the tissue by inserting the probe into body tissue. Because the probe is in the form of a needle, it is about 5 mm in diameter. Reducing its diameter to less than 1 mm in diameter would be difficult, yet it would still be too large to insert into body tissue and lose non-invasiveness completely. Further, because the probe is inserted into body tissue, strict safety measures would have to be followed.
As previously pointed out, a needle-type ultrasonic microscope, in which an ultrasound transducer is installed, suffers the disadvantages of being both invasive and difficult to use with assured safety.
On the other hand, an intraductal ultrasonic device [Japanese Unexamined Patent Application No. 2001-198127] and an ultrasonic treatment device [Japanese Unexamined Patent Application No. 2002-153483] both use fused quartz fibers. In these devices, the ultrasonic wave is transmitted along a fused quartz fiber. They differ intrinsically from the present invention, which operates as an ultrasonic microscope using, preferably, a fiber. The inventions described above have the drawbacks in non-invasiveness, and they do not improve a spatial resolution (of the images), because they use the low-frequency ultrasonic waves and do not enable the operator to control minutely the position of the tip of the fiber.
Japanese Unexamined Patent Application No. 2003-116898 discloses an ultrasonograph and an ultrasonic applicator for therapy. The objective of this invention is to transmit a high-intensity ultrasound beam using fused quartz fibers placed inside the ductal organ to the tip of the fiber to treat the region of interest: bladder stones, for example.
Here, a bundle of fibers transmits a high-power ultrasound beam. Further, according to reference 6, an ultrasound beam with a frequency as low as 1 MHz was used. The invention covered by reference 6 differs completely from the present invention, which enables the operator to obtain imaging at the cell level, showing the actual cell structure.
The invention disclosed by reference 6 has the drawbacks of not being non-invasiveness and of having low spatial resolution.    Reference 1: Japan Unexamined Patent Application No. H02-107238 (A11)    Reference 2: Japan Unexamined Patent Application No. H02-1072389 (A11)    Reference 3: Japan Unexamined Patent Application No. H11-206759 (A11)    Reference 4: Japan Unexamined Patent Application No. 2001-198127 (A11)    Reference 5: Japan Unexamined Patent Application No. 2002-153483 (A11)    Reference 6: Japan Unexamined Patent Application No. 2003-116869 (A11)